The invention relates to a method for operating an amalgam lamp having a nominal power Pnominal, comprising a discharge space containing a filling gas or in which a lamp voltage Uoptimum designed for a maximum UVC emission is applied between electrodes or a lamp current Ioptimum designed for a maximum UVC emission flows between electrodes, wherein the discharge space is accessible for an amalgam deposit, which can be heated by a heating element, in which a heating current Iheating is conducted through the heating element.
For amalgam lamps, mercury in the form of a solid amalgam alloy is introduced into the discharge space. The bonding of the mercury in the amalgam acts against a release in the discharge space. This allows higher operating currents (and higher temperatures), so that in comparison with conventional low-pressure mercury vapor lamps, three to six times higher radiated powers and power densities can be achieved.
An operating mode of an amalgam lamp according to the generic type mentioned above is described in International patent application publication No. WO 2007/091187 A1. The amalgam lamp comprises a quartz glass tube, which is closed on both ends by crimped sections, through each of which a current feedthrough is installed into the discharge space to a coil-shaped electrode. One of the crimped sections is provided with a hollow space that is open to the discharge space and in which the amalgam is introduced. The solid amalgam is thus arranged outside of the discharge. It can be heated separately. For this purpose, a heating device is provided in the vicinity of the amalgam deposit, which heating device has its own current circuit and a temperature control. Preferably, the coil-shaped electrode is simultaneously the heating device for heating the amalgam.
Amalgam lamps are typically operated with power regulation, sometimes also current regulation, wherein the nominal power or the nominal current is designed for the optimal mercury concentration in the discharge space and the corresponding maximum UVC intensity.
In the operating mode with “constant current” the temperature of the coil-shaped electrode is kept constant, so that the amalgam deposit remains at an approximately constant temperature and, in this respect, a mercury vapor pressure that is optimum for the operation is specified. This applies, however, only as long as the outside conditions do not change. If outside temperature changes or through warming of the lamp—for example by placement in a tight space—there is however a slight increase in temperature in the area of the amalgam deposit, so that the amalgam lamp is no longer operating at its optimum operating point, this leads to a reduced power and light output.
Amalgam lamps are as a rule operated in the “constant power” operating mode by a power-regulated ballast. In this connection it is to be noted that, in conventional amalgam lamps, a maximum UVC power is produced at a mercury vapor pressure around 0.8 Pa. The optimum is shown schematically in FIG. 3, where the UVC emission (output) is plotted in relative units versus the mercury vapor pressure in [Pa].
It has now been shown that the lamp voltage changes with the mercury vapor pressure. This applies, above all, for amalgam lamps having a filling gas containing helium or neon. This dependency is shown schematically in the diagram of FIG. 4, in which on the left ordinate the lamp voltage U and on the right ordinate the lamp current I are recorded, each in relative units versus the mercury partial pressure pHg in [Pa]. The optimum operating current Ioptimum produces a mercury vapor pressure around 0.8 Pa. In the operating mode with constant power P, the lamp current I has a reciprocal relationship relative to the lamp voltage U (according to P=U×I). Therefore, in the power-regulated operation, each change of the lamp voltage (curve 2) is compensated by an opposite adjustment of the lamp current (curve 1). The lamp current, however, directly influences the temperature of the coil-shaped electrode and thus, accordingly, the temperature of the amalgam deposit and consequently, by the mercury vapor pressure, also the lamp voltage.
For example, if the lamp voltage falls, this is compensated by the ballast by increasing the current, which, in turn, increases the temperature of the amalgam deposit and the mercury vapor pressure, which leads, in turn, to a further reduction of the voltage. Also, in the reverse direction, increases in the lamp voltage thus produce a corresponding build-up effect.
Consequently, this system cannot be kept stable at the optimum operating point, as for example at a mercury vapor pressure of 0.8 Pa.